1. Field of the Invention
The present invention relates to a mobile radio system, and more particularly, to a mobile radio system where a mobile station is in a standby state for a call from a base station over a predetermined paging channel.
2. Description of the Related Art
With conventional mobile radio systems, a predetermined radio channel is assigned to a paging channel (hereinafter, referred to as PCH). The mobile stations intermittently receive the paging channel PCH for a paging message from the base station over the PCH, and carry out a predetermined signal processing.
In particular, with the conventional portable telephone system, air interface requirements STD-27 of RCR (Research & development Center for Radio systems) specifies that the system incorporates a plurality of PCHs. But each mobile station is assigned only one PCH.
For example, if the number of radio carrier frequencies Nc is 3, the number of groups assigned to one radio carrier frequency NP is 3, and the total number of groups Nc.times.NP is 9, there exist a total of 9 logical channels (PCH1 to PCH9).
Mobile stations are in a standby state in a PCH which is determined by the following equation.
{(two least significant digits of mobile station identification (hereinafter, referred to as MSI)) -1}.div.9+1.
For example, a mobile station having a mobile station identification MSI=109 is assigned PCH9 as a paging channel and cannot be in a standby state on a channel other than PCH9.
It is to be noted that mobile stations of the portable telephone system are in a standby state most of the time, i.e., waiting for a call incoming.
Therefore, the mobile stations consume most of power in the standby state.
For prolonged battery life, the mobile stations are operated in an intermittent reception mode to reduce power consumption.
Long intervals between receptions can save the battery life significantly but it takes longer time before the mobile station can answer.
For example, if the mobile station monitors the PCH every 30 seconds, the mobile station may be in a standby state for a length of time ranging from 0 to 30 seconds and 15 seconds on average, depending on the timing at which the calling message is transmitted in the standby period of the mobile station.
If the mobile station receives the PCH every second, the mobile station needs to be in the standby state only for 0.5 seconds on average.
However, the mobile station consumes about 30 times more power in an intermittent reception mode of one-second intervals than in an intermittent reception mode of 30-second intervals.
The aforementioned conventional mobile radio system suffers from a problem that the whole system uses the same intermittent reception periods or intervals. Therefore, if the mobile stations are used frequently, a longer time for paging the mobile stations is needed. If the mobile stations are not used frequently, they consume more power.
The present invention is to solve the aforementioned drawbacks.